The present invention relates to a process for preparing 1,4-diketopyrrolo[3,4-c]-pyrrole pigments in microreactors.
1,4-Diketopyrrolo[3,4-c]pyrrole pigments are known and can be used for pigmenting macromolecular organic materials. They have acquired immense industrial importance for pigmenting paints in particular. Their fastnesses and color properties therefore have to meet stringent requirements in commercial practice. This is why the production process is immensely important.
Production processes are described for example in U.S. Pat. Nos. 4,415,685 and 4,579,949. U.S. Pat. No. 4,579,949 describes a conventional batch process for preparing 1,4-diketopyrrolo[3,4-c]pyrrole pigments by reaction of dialkyl succinates with aromatic nitrites in the presence of strong bases in alcoholic solution or suspension and subsequent hydrolysis of the resultant salt.
EP-A-0 640 603 discloses a batch process for preparing highly transparent 1,4-diketopyrrolo[3,4-c]pyrrole pigments.
EP-A-0 672 729 discloses a batch process for preparing hiding 1,4-diketopyrrolo[3,4-c]pyrrole pigments.
EP-A-0 962 499 discloses a batch process for preparing 1,4-diketopyrrolo-[3,4-c]-pyrrole pigments using crystal growth inhibitors during the synthesis.
A feature common to these processes is the need to control the process parameters. For example, temperature at and duration of the addition of the individual reactants, supplementary stirring times and temperatures, hydrolysis temperature, suspension concentration during reaction and hydrolysis, use of further solvents (such as water and/or alcohols) and of acid in the hydrolysis are decisive for the fastnesses, the color properties of the pigments obtained and their quality constancy. A particular requirement is the need to exclude even traces of water until the hydrolysis stage. Moreover, the scale-up of new products from the laboratory scale to the large industrial scale is inconvenient with batch processes and can present problems, since for example vessel and stirrer geometries or heat transfers have a substantial effect on particle size, particle size distribution and color properties.
It is an object of the present invention to provide an environmentally friendly, economical and technically reliable process for preparing diketopyrrolopyrrole pigments that provides optimal mixing of the reactants and very constant adherence to the desired process parameters and simplifies the scale-up.
It is known to conduct certain chemical reactions in microreactors. Microreactors are constructed from stacks of grooved plates and are described in DE 39 26 466 C2 and U.S. Pat. No. 5,534,328. It is pointed out in U.S. Pat. No. 5,811,062 that microchannel reactors are preferably used for reactions that do not require or produce materials or solids that would clog the microchannels.
It has now been found that, surprisingly, microreactors are useful for preparing diketopyrrolopyrrole pigments.
As used here, the term xe2x80x9cmicroreactorxe2x80x9d is representative of micro- and minireactors which differ only by reason of the dimensions and construction of the reaction channel structures.
It is possible to use, for example, microreactors as known from the cited references or from publications of the Institut fxc3xcr Mikrotechnik Mainz GmbH, Germany, or else commercially available microreactors, for example Selecto(trademark) (based on Cytos(trademark)) from Cellular Process Chemistry GmbH, Frankfurt/Main.